THC-COOH “carboxy THC,” lingers in urine long after cannabis is consumed. By looking at the concentration of THC-COOH in wastewater, Stats Canada can get a more accurate read on the total mass of cannabis consumed.

Unsurprisingly, larger cities had higher levels of THC-COOH, partly because of the sheer size of their populations. This doesn’t mean more people consume cannabis in Montreal, Toronto and Vancouver, a smaller pool of heavy consumers can easily skew the results.

If you expected Vancouver to have the highest consumption, you would be very wrong. Vancouver actually consumed the least per capita.

The city with the highest cannabis consumption per capita is actually the smallest. Halifax consumed the most per capita at approximately 1,310 micrograms/person-week.